Many people probably thought this was an end to the affair. But now prize-winning environmental South African journalist, Tony Carnie (writing for his own newspaper, itself called "The Mercury"), reveals that toxic mercury sludge remaining at Thor's closed-down "mercury recovery" site * may be causing the poisoning of people, fish and flora dependent on one of Durban's biggest reservoirs.

However, Thor is not the only possible culprit. Eskom, the country's largest coal-burning electrical utility, is also in the frame; as are a local ferromanganese, cement and gold producers. So long as such operations continue across the world, the curse of mercury pollution can never be completely laid to rest.

*Thor has since been renamed " Guernica Chemicals" - a name universally associated with the eponymous painting by Pablo Picasso depicting the horrendous Nazi bombing of this Spanish town in 1937.

Evidence of mercury poisoning in people, fish and plants around one of Durban's biggest drinking water reservoirs has sparked more questions than answers about the scale and source of the pollution.

Two weeks ago it emerged that unusually high levels of this brain and organ-damaging heavy metal had been found in the food chain and in hair samples collected from dozens of people living around Inanda Dam by the Medical Research Council.

Yet this latest evidence of poisoning should come as no surprise to the government, given that British and German researchers sounded a warning17 years ago about the future risk of fish poisoning at Inanda.

The initial finger of suspicion pointed towards the British-owned Thor Chemicals group, which handled large quantities of mercury for several decades at the nearby town of Cato Ridge.

Yet the subsequent discovery of higher than normal mercury levels in fish at Nagle Dam has blurred the picture and raised the possibility of multiple sources of mercury pollution, other than Thor.

Is it possible, for example, that the Assmang manganese factory in Cato Ridge may also be partly responsible for the pollution? Or has airborne pollution from Eskom's power stations, from cement factories and from other industries much further afield also fed the problem?

Other theories being investigated by a government task team include the possibility of mercury pollution from small-scale gold mines from days gone by, or naturally high levels of mercury in the soil.

And finally, is it possible that the waterlogged soils underneath Inanda Dam and nearby Nagle Dam have released mercury into the environment which has been eaten by fish and magnified up the food chain?

Some recent research studies reviewed by the World Health Organisation suggest that fish in newly created dams may become unsafe for human consumption for between 15 and 40 years. These studies, in Bolivia, Brazil, Canada and the United States, suggest that when land is flooded to create new dams, mercury is released from the sodden soils and converted to a more toxic form (methyl mercury).

Last week, provincial environment affairs spokesman Thami Ngidi confirmed that the task team was casting its net wider by including airborne and waterborne pollution from other industries - including emissions from the cement, coal and ferromanganese industries.

"We believe that there are some industries with the potential to contribute to the unusually high levels of mercury at Inanda, based on previous work which has been done. It is quite possible that these emissions may have been carried by wind or water," he said.

Ngidi did not mention any industries by name, but Eskom burns huge volumes of coal at its power stations in Mpumalanga province and the World Health Organisation has identified coal-fired power stations as the largest source of mercury pollution through the air.

The WHO also says the cement, gold, iron, steel and ferromanganese industries also generate mercury pollution.

One of the first warnings about future problems in the Inanda Dam came from Greenpeace researchers, who warned in 1991 that fish in the new dam would have to be monitored carefully for signs of mercury pollution.

Ten years later, other researchers confirmed the first signs of mercury build-up in carp and catfish collected at Inanda Dam. Graham Barratt and Jane Combrink, of the Durban University of Technology, said in 1999 that although the mercury levels in fish were below the South African safe consumption guidelines, it would be wise for Inanda Dam residents to cut their staple diet of fish by half.

In February 2006, Barratt approached The Mercury to register his surprise that no government departments appeared to have taken any serious interest in the results of his study.

He had delivered copies to former provincial health director-general Ronald Green-Thompson and also contacted senior officials of the departments of water affairs and environment affairs, as well as Umgeni Water.

Finally, in 2007, someone did at last start to listen. A team of MRC researchers led by Prof Angela Mathee and PhD student Vathiswa Papu went to Inanda to collect fish samples, mud samples and pieces of hair from86 people living close to the dam.

One very high mercury reading was found in a mud sample close to the old Thor Chemicals site, along with much lower readings in mud collected from the Umgeni River and Inanda Dam.

The researchers collected only 10 fish samples for analysis - yet 50% of these fish had mercury readings above the safe consumption limits recommended by South African law and the WHO.

The human hair samples collected from people in Mshazi, Nqetho and Madimeni villages also produced several surprises.

Almost 20% of the samples contained mercury residues above the WHO recommended limit of 7ug/g. Two people had mercury levels above 50ug/g which is classified as an "intoxicating" level which threatens health.

It is understood that both people were examined at King Edward VIII Hospital but did not show obvious clinical symptoms of mercury poisoning or neurological damage.

Nevertheless, Mathee and Papu were surprised by the results and compiled a preliminary report to alert government agencies to a potentially serious health threat to people living next to Inanda Dam.

"We did not expect to find anything, but it turned out to be a small study with big results," Mathee said.

She stressed that the results were preliminary and still had to peer-reviewed - yet they indicated the need for a thorough follow-up investigation to find out how many people were affected, as well as the exact geographic boundaries and source of the problem.

The preliminary results of the MRC report were presented to officials in January, prompting the establishment of a task team involving several government departments, Umgeni Water and the eThekwini Municipality.

Since then several further samples have been collected from Inanda Dam, local rivers, the Wiggins water treatment works and Nagle Dam.

According to a statement from task team chairman Dr Timothy Fasheun, there is no risk to public health from drinking water samples collected from the Wiggins treatment works (which purifies Inanda Dam raw water).

Fasheun said irrigation water from Inanda dam was also safe to use - but higher than expected mercury contamination had been found in fish and vegetables at Inanda and Nagle dams.

The discovery of elevated mercury levels in Nagle Dam (which lies beyond the surface water catchment footprint of Thor Chemicals) suggested that there could be other unknown sources of mercury pollution.

***

Two possible industrial pollution sources operate side by side

October 27, 2008 Edition 1

BASED on the available evidence, one of the most obvious sources of mercury pollution close to Inanda is the British-owned Thor Chemicals (now called Guernica Chemicals) which set up shop in South Africa in 1963.

It began producing mercury products at Cato Ridge in the late 1970s and by the early 1980s built a plant to recycle mercury waste products. Thor imported sludge and other mercury waste from all corners of the world (including Indonesia, Italy, the United States, Britain and Brazil).

But its mercury "recovery plant" was unable to process the huge volumes which arrived at Cato Ridge, and in 1997 a government commission of inquiry chaired by Prof Dennis Davis estimated that Thor had a stockpile of more than 3 000 tons of mercury waste (most of this stockpile is still stored at Thor today, pending a final decision on how to get rid of it safely).

Later reports showed that mercury waste was dumped into the soil or unlined evaporation ponds. Mercury also leaked out of rusty storage drums through cracked concrete floors and by 1994 the soil was polluted to a depth of more than a metre.

But mercury had begun to escape into the wider environment at least a decade earlier. In 1988, Umgeni Water discovered high levels of mercury in a stream close to Thor which leads into the Umgeni River and then Inanda Dam.

Later studies showed traces of mercury on the banks of the Umgeni River, nearly 10km away from Thor, but the highest levels were much closer to the factory.

Another possible industrial source is Thor's next-door neighbour in Cato Ridge - the Assmang ferromanganese factory, which has been operating since 1959.

A massive dump containing more than five million tons of slag (waste products) has accumulated at Assmang over the past four decades - yet according to a recent report by SRK Consulting Engineers and Scientists, the slag heap was not separated from the soil by a protective lining. Nor was there any formal drainage system to capture slag dump pollutants which drained into the surrounding soil and rivers.

"All water run-off from Assmang is therefore ultimately discharged into the Inanda Dam," says the report, noting that 570 tons of slag is dumped from the factory every day and that a separate study by the consulting group Geomeasure has found evidence of toxic chemical contamination of nearby surface water and groundwater. It is not clear what effort was made to investigate the presence of mercury in the Assmang slag dump, groundwater or chimney stack emissions, although some mercury readings were detected in slag samples by consultants Moore Spence Jones earlier this year.

There is also evidence from other parts of the world that ferromanganese factories can generate significant quantities of mercury.

For example, managers of the Tinfos manganese plant in Norway discovered in 1993 that they were pumping about 100kg of mercury into the air from the chimney stacks every year because local manganese ore and other raw materials contained significant levels of mercury.

A spokesman for Guernica Chemicals S A (Pty) Ltd said the company had not seen the MRC report, nor had it been approached for any information or comment by any authority.

Jan Steenkamp, the executive director of Assmang Limited, said: "Assmang is of the opinion that its operations cannot be linked to reports of higher than recommended levels of mercury in the environment of the Inanda and Nagle dams, both of which are located many kilometres from our Cato Ridge Works."